Some have the custom not to eat meat… during the week [of 9 Av, until after 9 Av]; some add from rosh chodesh until the fast; some add from 17 Tamuz.

Rama holds of the middle view. He adds:

We hide the slaughtering knife from rosh chodesh, since we don't slaughter (except for a mitzva, as for a sick person, the sabbath, a mila, or the like).

Nowadays, obviously, we don't need to slaughter animals right before eating them. Moreover, many non-Jews buy kosher meat. The above-implied reason for not slaughtering during the nine days is pretty much gone.

What is the current common practice? Are kosher slaughterhouses open during the nine days? during the week of 9 Av?

Are there any pos'kim that explicitly permit or ban slaughtering at this time of year in light of modern technology, infrastructure, and/or demand?